Thermal processing of semiconductor wafers is common in the semiconductor manufacturing industry. Rapid thermal processing (“RTP”) is one common type of thermal processing. RTP involves rapidly heating a semiconductor wafer inside a chamber. The semiconductor wafer may be sliced from a single crystal ingot grown by a method such as the Czochralski method. RTP involves fast temperature ramp rates. For example, a typical RTP system is capable of heating a wafer to about 1200 degrees centigrade or more from room temperature in just seconds. The temperature ramp rates may be as fast as about 200 degrees centigrade per second or more. In some instances, the heating is provided by high-power incandescent lamps. Thermal processing systems using hot plates or microwaves to heat the wafer are also available. The RTP system may “spike anneal” the wafer by rapidly cooling the wafer after heating, also minimizing the dwell time at peak temperature.
Conventional RTP systems treat the wafer while it rests on a wafer holder inside a process chamber. Commonly, the wafer holder comprises support pins that hold the wafer in an elevated position in the chamber. In some RTP systems, the wafer remains stationary during the RTP treatment, which may be controlled by switching the heat source on and off. Alternatively, the heat source may provide continuous heating, in which case the wafer holder moves the wafer in close proximity to the heat source for heating and then moves the wafer away from the heat source for cooling. RTP has been used for a wide variety of purposes, including forming source and drain contact junctions, shallow extension junctions, and electrically active polycrystalline silicon gate electrodes, to name just a few. RTP may also be used to improve internal gettering in silicon wafers as described in U.S. Pat. No. 6,361,619 (Falster), which is incorporated herein by reference.
The semiconductor industry has had a growing demand for wafers having fewer and fewer defects. The areas of the wafer adjacent the support pins during thermal processing treatments, such as RTP, have been found to have more defects than other areas of the wafer. It has been theorized that mechanical stresses introduced by the support pins contribute to the formation of dislocation defects on the back of the wafer, which can then propagate to the front surface of the wafer where they degrade semiconductor devices fabricated on the front surface of the wafer. Thus, semiconductor manufacturers have attempted to reduce the mechanical stresses introduced by the support pins.
Semiconductor manufacturers have also tried to reduce or eliminate uneven heating of the wafer caused by the support pins. Uneven heating causes thermal stresses in the wafer, which may introduce defects to the wafer. In some RTP systems, such as those that heat the wafer with incandescent lamps, the wafer may be heated more than the support pins. Consequently, the cooler support pins may conduct heat away from the wafer, resulting in localized cooling in the vicinity of the support pins. In other wafer thermal processing systems, such as those in which the wafer is supported above a heated susceptor or hot plate, the support pins may become hotter than the wafer. When this happens, heat conducted to the wafer through the support pins may present a thermally opposite situation.
Efforts to reduce uneven heating of the wafer caused by the support pins have been directed to minimizing contact between the support pins and the wafer. For example, U.S. Pat. No. 6,214,122 (Thompson) discloses support pins having “minimum contact points” to reduce mechanical contact between the contact points and the wafer. In other words, the support pins have very sharp points so there is only a small contact area between the support pins and the wafer. Minimizing the contact area between the wafer and the support pins is intended to produce more uniform wafer heating and reduce thermal stresses in the wafer by reducing the conductive heat transfer between the support pins and the wafer.